The immune system of a healthy new-born infant is not as effective as that of an older healthy child or a healthy adult. To a great extent, this is because the newborn infant has yet to encounter potential antigens and the B cells and T cells have yet to mature such that they can mount appropriate immune responses. The newborn infant is not completely unprotected against pathogens because maternal antibodies of the IgG class cross the placenta during pregnancy and remain in the infant for several months. Further, the colostrum produced in the first few days after birth and the milk which succeeds it are rich in maternal antibodies of the IgA class. This natural passive immunity in effect “buys time” for infants who encounter common pathogens in the first few weeks after birth as the maternal antibodies confer a degree of immunity on the baby whilst the baby's own immune system is activated.
However, the immune system of some otherwise healthy neonates may not be fully mature at birth with the result that the infant will be even slower starting to mount its own immune responses even than an infant born with a fully mature immune system. This problem is seen at its most severe in preterm infants. Such infants may be considered to be immune-compromised to some degree. Immune-compromised subjects are in general at risk of infection by opportunistic pathogens such as Pseudomonas aeruginosa which may cause infections of the urinary and respiratory tracts for example.
Further, although mother's milk is recommended for all infants in some cases breast feeding is inadequate or unsuccessful for medical reasons or the mother chooses not to breast feed. Infant formulas have been developed for these situations.
In the recent past, certain strains of bacteria have attracted considerable attention because they have been found to exhibit valuable properties for man if ingested. In particular, specific strains of the genera Lactobacilli and Bifidobacteria have been found to be able to colonise the intestinal mucosa, to reduce the capability of pathogenic bacteria to adhere to the intestinal epithelium, to have immunomodulatory effects and to assist in the maintenance of well-being. Such bacteria are sometimes called probiotics. They may be incorporated in nutritional products such as infant formulae.
Extensive studies have peen carried out to identify new probiotic strains. For example, EP 0 199 535, EP 0 768 375, WO 97/00078, EP 0 577 903 and WO 00/53200 disclose specific strains of Lactobacilli and Bifidobacteria and their beneficial effects.
Recently, Matsumoto et al have reported that certain probiotic bacteria, particularly Bifidobacteria were effective in protecting mice against gut-derived sepsis caused by Pseudomonas aeruginosa (J. Appl. Microbiol, 2007). The authors comment that this discovery may offer the possibility of developing an alternative therapy to antibiotics, particularly given that bacteria such as P. aeruginosa are becoming increasingly resistant to antibiotics.
From the foregoing, it may be seen that there is a need for an effective method for the prevention of opportunistic infections in immune-compromised subjects, particularly premature and neo-natal infants which does not rely on the use of antibiotics and which may be conveniently and safely administered.